Device and method for diagnosing the condition of a probe upstream from a catalytic converter

ABSTRACT

An apparatus and method for diagnosing the condition of a sensor in an internal combustion engine upstream from the catalytic converter. The diagnosis utilizes a signal from a second non-linear probe downstream from the catalytic converter. This signal is processed to give a signal that is filtered. The filtered signal is in turn compared with maximum and minimum values. The upstream probe is considered to be correct if the signal falls between these two values or faulty if it falls outside the values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to internal combustion engines of thefuel-injection type equipped with a catalytic exhaust converter precededby a sensor and, more particularly in such engines, a device and aprocess for diagnosis of the condition of the sensor disposed upstreamfrom the catalytic converter.

2. Discussion of the Background

It is known how to use systems for modifying the quantity of fuelinjected into an engine as a function of the exhaust-gas compositionand, more particularly, of the oxygen content of these gases. To thisend, the oxygen content is measured by means of a nonlinear sensor knownas the “lambda” sensor or EGO sensor, where EGO is an English-languageacronym for “Exhaust Gas Oxygen”. Such a sensor is disposed upstreamfrom the catalytic exhaust converter, and the signal delivered by thissensor is used to modify the quantity of fuel injected into the enginecylinders via a first feedback loop. For this reason, the sensor is alsoknown as a richness-regulating sensor.

It is clear that poor condition of this sensor leads to poor operationof the engine and of the catalytic converter, in turn leading topollutant emissions at abnormally high levels. It is therefore importantto determine the condition of this sensor at all times in order todiagnose poor operation thereof when its condition has deterioratedbeyond certain limits. The present solutions for diagnosis of thecondition of the upstream sensor comprise analyzing the behavior of thesensor in response to richness excitations in open loop or closed loopand monitoring the following parameters:

the minimum voltage delivered by the sensor: if too high, a fault isindicated;

the maximum voltage delivered by the sensor: if too low, a fault isindicated;

the lean-to-rich transition time; if too long, a fault is indicated;

the rich-to-lean transition time; if too long, a fault is indicated;

the period of the signal delivered by the sensor in closed loop: if toolong, a fault is indicated.

The diagnosis then comprises declaring failure of the sensor if one ormore faults are detected.

Such a diagnostic process is based on analysis of the sensor behavior inorder co deduce therefrom a sensor condition on the basis of assumeddegradation mechanisms. For example, as a sensor ages, its dynamicvoltage range is reduced and/or its transition times become longer Thedisadvantage of such a diagnostic process is that a perfect correlationdoes not exist between these measurements and the emissions ofpollutants.

In addition, calibration of fault detection thresholds proves to be verytricky and necessitates:

perfect knowledge of the mechanisms of aging of the sensors,

numerous tests to establish a relationship between the measureddegradations of parameters and their effects on pollutant emissions.

In addition, it is not possible in all cases to guarantee that thediagnosis is reliable. For example, a sensor with reduced dynamicvoltage range may prove to be good with regard to pollutant emission ifonly that characteristic is affected.

SUMMARY OF THE INVENTION

One object of the present invention is therefore to provide, fordiagnosis of the condition of a sensor disposed upstream from acatalytic converter associated with an internal combustion engine of thefuel-injection type, a device and a process which do not exhibit theaforesaid disadvantages of the devices and processes of the prior art.

Another object of the present invention is also to provide, fordiagnosis of the condition of an upstream sensor, a device and a processwhich does not depend on measurements of intrinsic characteristics ofthe sensor. The process of the invention is based on monitoring ofcharacteristics of the richness feedback loop which have an influence onpollutant emission, or in other words the mean period and mean richnessof the feedback loop. In this way, the condition of the upstream sensoris evaluated on the basis of effects that it produces on the richnessfeedback loop, or in other words on the emissions of pollutants, and noton the basis of its intrinsic characteristics.

The effects of the condition of the upstream sensor are capable ofcausing pollutant emissions by exceeding the limits of the “window” ofgood operation of the catalytic converter, this exceeding being due todrift of the mean operating richness and/or to excessively long meanperiod of the richness loop.

To detect drift of the mean operating richness, the invention proposesto provide a second nonlinear sensor disposed downstream from thecatalytic converter and constituting an integral part of a secondfeedback loop, by virtue of which the output voltage V_(downstream) ofthe second sensor, called downstream sensor hereinafter, is slaved to asetpoint voltage VC_(downstream) corresponding to the center of thewindow of good operation of the catalytic converter. The signaldelivered by this loop is used to modify the signal of the firstfeedback loop containing the upstream sensor.

Such a system of richness slaving with double control loop is describedin the patent application filed today by the Applicant and entitled:“SYSTEM AND PROCESS WITH DOUBLE CONTROL LOOP FOR INTERNAL COMBUSTIONENGINE”. The invention relates to a device for diagnosis of thecondition of a nonlinear sensor disposed upstream from a catalyticconverter associated with an internal combustion engine of thefuel-injection type controlled by an electronic computer, the saidengine containing a first control loop, including the said nonlinearsensor, to deliver to the computer a first signal KCL for correction ofthe quantity of fuel injected, and a second control loop, including asecond nonlinear sensor disposed downstream from the said catalyticconverter, to deliver a second signal KRICH for correction of thequantity of fuel injected, the said diagnostic device beingcharacterized in that it comprises:

a filter circuit to which there is applied the second correction signalKRICH in order to deliver a filtered signal KRICH_(F),

a measuring circuit to which there is applied the output signalV_(upstream) of the upstream sensor in order to determine the mean valueT_(m) of the period of correction of the first control loop, and

a logic circuit to determine, as a function of the values of thefiltered signal KRICH_(F) and of the mean period T_(m), whether thecondition DIAG of the upstream sensor is good or defective.

In one embodiment of the invention, the logic circuit determines thatthe upstream sensor is defective if the filtered signal is larger than amaximum value or smaller than a minimum value or else if the mean periodis longer than a maximum value.

In another embodiment of the invention, the maximum and minimum valuesof the filtered signal KRICH_(F) are determined by calibration as afunction of the value of the mean period and are stored in a memory.This memory is addressed by the value of the mean period in order todeliver the maximum and minimum values, with which the value of thefiltered signal is compared.

The invention also relates to a process which comprises the followingstages:

filtering of the second correction signal KRICH to obtain a filteredsignal KRICH_(F),

calculation of the mean value T_(m) of the period of the output signalV_(upstream) of the upstream sensor,

comparison of the said filtered signal KRICH_(F) with two values, themaximum KRICH_(max) and the minimum KRICH_(min), to determine whetherthe condition DIAG of the said upstream sensor is correct or defective,according to whether the filtered signal KRICH_(F) is respectivelywithin the limits defined by the maximum and minimum values or outsidethe said limits for the value of the mean period T_(m).

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the present invention willbecome apparent upon reading the following description of a particularembodiment, the said description being made with reference to theattached drawings, wherein:

FIG. 1 is a functional diagram of a system for double-loop control ofrichness to which the invention applies;

FIGS. 2-A and 2-B are diagrams showing how the richness correction isapplied with a single feedback loop containing one sensor upstream fromthe catalytic converter;

FIGS. 3-A and 3-B are diagrams showing one mode of correction of therichness by using a second feedback loop containing a sensor downstreamfrom the catalytic converter;

FIG. 4 is a diagram showing the mode of filtering of the correctionsignal KRICH to obtain a filtered signal KRICH_(F);

FIG. 5 is a diagram showing an algorithm for calculation of the meanperiod of the signal of the upstream sensor;

FIG. 6 is a diagram showing the curves which define the zones of corrector defective functioning of the upstream sensor, and

FIG. 7 is a diagram showing a decision algorithm for determining thecondition of the upstream sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, an internal combustion engine 10 is controlled in knownmanner by an electronic computer 12. The exhaust gases of this engineare filtered by an exhaust muffler 14 of the catalytic converter type,from which they escape to the open air. A first sensor 16 is disposed atthe inlet of the exhaust muffler and measures the content of one of themain components of the exhaust gases, this component usually beingoxygen. This sensor is of the nonlinear type, and is often called, asindicated hereinabove, a “lambda” sensor or EGO sensor. This sensordelivers at its output terminal an electric signal V_(upstream) (FIG.2-A), which is applied to a comparator circuit 18 in which V_(upstream)is compared with a threshold voltage VS_(upstream) to determine the signof V_(upstream) relative to that threshold.

The threshold value VS_(upstream) depends on the sensor characteristicsand corresponds to the transition voltage of the sensor when theconditions of stoichiometry are satisfied.

The output terminal of comparator circuit 18, which delivers a binarysignal 1 or 0, is connected to the input terminal of a firstrichness-regulating correction circuit 20 of theproportional-plus-integral type with gains P and I respectively. Thecorrection circuit 20 delivers a signal KCL, which has the shaperepresented by the diagram of FIG. 2-B. It is this signal KCL which isdelivered to computer 12 to control the quantity of fuel to be injected.Thus, as soon as V_(upstream) becomes smaller than VS_(upstream), thismeans that the mixture is lean in fuel and that the quantity of fuelmust be increased. This is accomplished by the jump +P (FIG. 2-B)followed by a positive slope of value I until the instant thatV_(upstream) exceeds VS_(upstream), which means that the mixture hasbecome rich in fuel and that the quantity thereof must be reduced. Thisis accomplished by a jump −P followed by a negative slope of value I.

The correction value KCL delivered by correction circuit 20 is modifiedby a second correction circuit 22, which introduces a correction termKRICH before being applied to computer 12. This correction term KRICH isdetermined by a circuit 24 on the basis of an output signalV_(downstream) of a second lambda sensor 26, which is disposed at theoutlet of the catalytic exhaust converter 14. This circuit 24substantially comprises a comparator 28, to which there are applied thesignal V_(downstream) and a setpoint signal denoted by VC_(downstream),and a third correction circuit 30, to which there is applied the signal(V_(downstream)−VC_(downstream)) delivered by comparator circuit 28. Thethird correction circuit 30 is, for example, of the proportional plusintegral type, and delivers the signal KRICH, which is applied to thesecond correction circuit 22.

The second correction circuit 22 is able to introduce the correctionKRICH by different modes, one of which will be explained with referenceto the timing diagrams of FIGS. 3-A and 3-B. These diagrams are plots ofthe signal KCL as modified by the second correction circuit 22, themodified signal KCL being denoted by KCL_(m).

According to the diagrams of FIGS. 3-A and 3-B, the signal KRICH isapplied during lean-to-rich transitions detected by the first sensor,which corresponds to the descending side of the signal KCL. In the casein which KRICH>0 (increasing the richness), the plot of KCL_(m) is thatof FIG. 3-A, while in the case in which KRICH<0 (increasing theleanness), the plot of KCL_(m) is that of FIG. 3-B.

The device for diagnosis of the condition of sensor 16 comprises theelements represented inside the rectangle 40 of the diagram of FIG. 1.These are a filter 32, to which there is applied the output signal KRICHof correction circuit 24 of the second loop, as well as a circuit 34 forcalculation of the mean period T_(m) of the signal V_(upstream) of theupstream sensor 16. The output terminals of filter 32 and of calculationcircuit 34 are connected to a logic circuit 36, which determines whetherthe condition of sensor 16 is good or poor as a function of the outputsignal KRICH_(F) of filter 32 and of the value T_(m) of the mean periodof the signal V_(upstream). The binary signal 1 or 0 corresponding togood or poor condition of sensor 16 appears at the output terminal DIAGof logic circuit 36.

The communications delivered by computer 12 are as follows:

the engine speed REG,

the pressure P of the intake manifold,

the state of the first loop: active or inactive,

the state of the second loop: active or inactive.

Circuits 32 and 34 process the communications listed above and authorizefiltering and calculation of T_(m) only if the following conditions aresatisfied simultaneously:

REG_(min)<REG<REG_(max)

P_(min)<P<P_(max)

first loop in active state,

second loop in active state,

where REG_(min) and REG_(max) are respectively the minimum and maximumvalues of engine speed REG between which the diagnosis can be made;P_(min) and P_(max) are respectively the minimum and maximum values ofthe pressure P of the intake manifold between which the diagnosis can bemade. Filter circuit 32 performs the calculation of the filteredrichness correction KRICH_(F) according to the algorithm of FIG. 4. Thiscalculation (step 42) is performed only if the conditions listed aboveare satisfied (step 44) and, in this case, the mean richness KRICH_(F)is given by:

KRICH_(F)=KRICH_(F)+K(KRICH−KRICH_(F))

where K is a filter factor between 0 and 1.

Calculation circuit 34 performs the calculation of the mean period T_(m)according to the algorithm of FIG. 5. This calculation is performed onlyif the conditions listed above are satisfied (step 50). This calculationof the mean period T_(m) comprises counting the transitions of thevoltage V_(upstream) from a value smaller than the thresholdVS_(upstream) to a value larger than the threshold during a certain timeinterval T_(D) and dividing this interval T_(D) by the number N oftransitions that were detected. The algorithm for calculation of themean period T_(m) of the first loop is represented by the diagram ofFIG. 5. The first step (50) comprises verifying whether the diagnosticconditions listed above are satisfied. If the response is “YES”,counting step 52 for time T is started, or in other words thecalculation of the mean period T_(m) begins. As soon asV_(upstream)>VS_(upstream) (step 54) and the sensor's previous state,STATE_(A), corresponding to V_(upstream)<VS_(upstream) (STATE_(A)=0),step 58 comprises storing this new state of the sensor in memory asSTATE_(A)=1. The following step 60 comprises verifying whether atransition (TRANS=1) was already detected previously; if the response ispositive, this means that a period has elapsed and the count 62 of thenumber N of periods is incremented by one unit. At the same time, thecounter of the duration T_(D) of the diagnosis is incremented by thevalue T of the counter 52. The calculation 66 of the mean periodT_(m)=T_(D)/N is then performed with the new values of N and T_(D). Thefollowing step 68 resets counter 52 to zero for a new measurement T ofthe period in progress.

In order that the calculation described in the foregoing can beperformed correctly, the following states must be present:

TRANS=0, STATE_(A)=1 and T=0,

which is accomplished by steps 72, 74 and 76 in cascade, which areinitialized by the verification (step 50) that the diagnostic conditionsare not satisfied, which is always the case during starting of theengine. Thus, for the first measurement of the period, the counter 52 isat the value 0 but, since STATE_(A)=1, the calculation cannot beginuntil this state changes to STATE_(A)=0, in order to be certain ofdetecting a transition In the desired direction. This is obtained by thedetection that V_(upstream)<VS_(upstream), in which case the change toSTATE_(A)=0 takes place (step 78).

During starting, TRANS=0, and so the condition of step 60 is notsatisfied and the period cannot be calculated. Otherwise, step 70imposes TRANS=1, which resets counter 52 to zero via step 68, and a newcount of T can begin.

During starting, STATE_(A)=1, and so the condition of step 56 is notsatisfied, in which case the steps of the algorithm begin over again.

Logic circuit 36 performs the steps of the algorithm of FIG. 7 in orderto compare the value of KRICH_(F) with values determined as being thelimit values beyond which the sensor is considered to be defective,specifically for a determined value T_(m) of the mean period.

These limit values, denoted by KRICH_(max) for too large richnessincrease and KRICH_(min) for too large leanness increase, are determinedby calibration with the use of a series of sensors whose agingcharacteristics are known.

This calibration permits plotting of the curves KRICH_(max) andKRICH_(min) as a function of the period T_(m) (FIG. 6), and these curvescan be stored in memory in the form of two maps or of a single map thatconsolidates both curves. These maps can be constructed by memorieswhich are addressed by the value of Tm, and the values read areKRICH_(max) and KRICH_(min) corresponding to the value of T_(m) (FIG.6).

The first step 80 of the diagnostic algorithm comprises comparing theduration T_(D) for calculation of the period T_(m) to a minimum durationT_(Dmin), shorter than which a diagnosis would not be reliable. IfT_(D)>T_(Dmin), the following step 82 comprises comparing KRICH_(F) witha value KRICH_(max) read from the map 88 giving KRICH_(F)=S(T_(m)). Thismap is addressed by the value of T_(m) to obtain a value of KRICH_(max),which is compared with KRICH_(F). If the condition is not verified, thesensor is considered to be defective (step 92).

If the condition is verified, the following step 84 is to compareKRICH_(F) with the value of KRICH_(min) for T_(m) as read from map 86,in which there are stored the values of the curve KRICH_(min)=S(T_(m)).If the condition KRICH>KRICH_(min) is not verified, the sensor isconsidered to be defective (step 92), with DIAG=0. In the opposite case,the sensor is considered to be correct (step 90), with DIAG=1.

As soon as the sensor is considered to be correct or defective, thediagnosis is terminated (step 94) and a new diagnosis can be initiatedto obtain a new value of KRICH_(F) and of T_(m).

When the curves of FIG. 6 are reduced to the form of maps, and thealgorithm of FIG. 7 is applied, the sensors considered to be poor(DIAG=0) are in the shaded portion outside the two curves, and thesensors considered to be good (DIAG=1) correspond to the area betweenthe curves.

Instead of the two curves of FIG. 6, it is possible to limit the choiceto fixed thresholds for KRICH′_(max), KRICH′_(min) and T′_(max), and soit is no longer necessary to have two maps. In this simplified case, thevalue of KRICH_(F) is compared with the two chosen thresholds, while thevalue T_(m) of the mean value is compared with the threshold T′_(max).If KRICH_(F) is larger than KRICH′_(max) or smaller than KRICH′_(min) orlarger than T′_(max), the sensor is considered to be defective. In theopposite case, the sensor is considered to be good.

The algorithm of FIG. 7 can be implemented in the form of a softwareroutine or in the form of electronic circuits, in which the comparisonsteps 80, 82 and 84 would be accomplished by digital comparators.

What is claimed is:
 1. A device for diagnosis of the condition of anonlinear sensor disposed upstream from a catalytic converter associatedwith a fuel injected internal combustion engine controlled by anelectronic computer, the engine containing a first control loop,including said nonlinear sensor, to deliver to the computer a firstsignal for correction of a quantity of fuel injected, and a secondcontrol loop, including a second nonlinear sensor disposed downstreamfrom the catalytic converter, to deliver a second signal for correctionof the quantity of fuel injected, said diagnostic device comprising: afilter circuit to which there is applied the second correction signal inorder to deliver a filtered signal, a measuring circuit to which thereis applied an output of the upstream sensor in order to determine themean value of the period of correction of the first control loop, and alogic circuit to determine, as a function of the values of the filteredsignal and of the mean period, whether the condition of the upstreamsensor is good or defective.
 2. A diagnostic device according to claim1, wherein the filter circuit accomplishes first-order filtering.
 3. Adiagnostic device according to claim 2, characterized in that the filtercircuit is of digital type.
 4. A diagnostic device according to claim 2,characterized in that the circuit for calculation of the mean value ofthe correction period of the first control loop is of the digital type.5. A diagnostic device according to claim 2, characterized in that thelogic circuit comprises three comparators, the first of which comparesthe value of the filtered signal with a maximum value in a firstcomparator, the second compares the value of the filtered signal with aminimum value, and the third compares the value of the mean period witha maximum value, the upstream sensor being considered to be defectivewhen the value of the filtered signal is larger than the maximum valueor smaller than the minimum value or larger than the maximum value ofthe mean period.
 6. A diagnostic according to claim 1, wherein thefilter circuit is a digital filter.
 7. A diagnostic device according toclaim 6, characterized in that the circuit for calculation of the meanvalue of the correction period of the first control loop is of thedigital type.
 8. A diagnostic device according to claim 6, characterizedin that the logic circuit comprises three comparators, the first ofwhich compares the value of the filtered signal with a maximum value ina first comparator, the second compares the value of the filtered signalwith a minimum value, and the third compares the value of the meanperiod with a maximum value, the upstream sensor being considered to bedefective when the value of the filtered signal is larger than themaximum value or smaller than the minimum value or larger than themaximum value of the mean period.
 9. A diagnostic device according toclaim 1, wherein the measuring circuit for calculation of the mean valueof the correction period of the first control loop is a digital circuit.10. A diagnostic device according to claim 9, characterized in that thelogic circuit comprises three comparators, the first of which comparesthe value of the filtered signal with a maximum value in a firstcomparator, the second compares the value of the filtered signal with aminimum value, and the third compares the value of the mean period witha maximum value, the upstream sensor being considered to be defectivewhen the value of the filtered signal is larger than the maximum valueor smaller than the minimum value or larger than the maximum value ofthe mean period.
 11. A diagnostic device according to claim 1, whereinthe logic circuit comprises three comparators, a first of which comparesthe value of the filtered signal with a maximum value , a second ofwhich compares the value of the filtered signal with a minimum value,and a third of which compares the value of the mean period with amaximum value of the mean period, the upstream sensor being consideredto be defective when the value of the filtered signal is larger than themaximum value or smaller than the minimum value or larger than themaximum value of the mean period.
 12. A diagnostic device according toclaim 11, wherein the logic circuit comprises at least one map or memoryin which there arc stored the maximum and minimum values of the filteredsignal as a function of the value of the mean periods and twocomparators, a first of which compares the value of the filtered signalwith a maximum value read from the map, and a second of which comparesthe value of the filtered signal with a minimum value read from the map,reading from the map being accomplished by means of the mean period. 13.A process for diagnosis of the condition of a nonlinear sensor disposedupstream from a catalytic converter associated with a fuel injectedinternal combustion engine controlled by an electronic computer, theengine containing a first control loop, including the nonlinear sensor,to deliver to the computer a first signal for correction of a quantityof fuel injected, and a second control loop, including a secondnonlinear sensor disposed downstream from the said catalytic converter,to deliver a second signal for correction of the quantity of fuelinjected, the diagnostic process comprising the steps of: filtering thesecond correction signal to obtain a filtered signal, calculating themean value of the period of the output signal of the upstream sensor,comparing the filtered signal with predetermined maximum and minimumvalues of the filtered signal, to determine whether the condition of theupstream sensor is correct or defective, according to whether thefiltered signal is respectively within the limits defined by the maximumand minimum values, or outside predetermined limits for the value of themean period.
 14. A diagnostic process according to claim 13, furthercomprising the steps of: calibrating to determine maximum and minimumvalues for a plurality of values of the mean period, storing saidmaximum and minimum values as well as values of the mean period in amemory addressable via its contents, and reading said memory by means ofthe mean value of the period to obtain the maximum and minimum values ofthe mean period.